A thesis submitted to the School of Graduate Studies in partial fulfillment of the requirements of the degree of

Investigation of Integrated Geophysical Methods to Characterize Near Surface Formations for Environmental Engineering

by

© Bilal Hassan

A thesis submitted to the School of Graduate Studies in partial fulfillment of the requirements of the degree of

Doctor of Philosophy in Civil Engineering Faculty of Engineering and Applied Science

Memorial University of Newfoundland

May, 2017

St. John’s Newfoundland and Labrador

ABSTRACT

Near surface nondestructive imaging for is aimed at evaluating unconsolidated to moderately consolidated porous or granular sediments. Usual targets of interest are depth to bedrock, depth of water table and mapping of subsurface morphology and evolution of toxic fluid (spills) flows such as oils, concentrated brines and their interactivity. Investigative propositions of interest span vast realm of geo- environmental engineering. This includes such applications as structural health and aseismic monitoring, hydraulic/hydrogeological characterization, geotechnical investigations including critical zones/sites identification in areas of public and industrial infrastructure development, groundwater management and natural resources exploration and exploitation associated activities such as mining and those termed unconventional in oil and gas industry. Conventional field methods include P- and S- wave surveys, and electrical resistivity (resistance) measurements. The presented results are outcomes and findings of two laboratory experimental studies designed, owing to their inherent amenability, on field scale concepts of the said methods, to advantage. In the first of two studies S-wave polarized propagation characteristics are exploited to quantitatively evaluate the combined architectural, rheological and fluid transport properties effect of fractured porous media (sandstone specimen) upon acquired or recorded (ultrasonic) S-wave signature when through transmitted with controlled source pulsing. Various time and frequency based analyses unambiguously delineate fracture geometry, stress effects of fracture stiffness and density, amplitude effects of fracture aperture size against the stationary effects of azimuth variability.

Critical findings include the characteristic stop-band artifact in transmitted bandwidth signature of fracture planes and direct correlation of S-wave velocity anisotropy with permeability anisotropy. The second study involved spatio-temporal imaging of an immiscible fluid displacement (oil with brine) through an unconsolidated granular sediment analogue (glass-beads-pack) under controlled constant head flow conditions against gravity, using P- , S-wave and electrical resistance data, acquired as integrated. Dry and saturated granular material characteristics with different saturant (state) effects were evaluated post analyzed integrated offering fresh insights.

Peculiar repeatable artifacts in imaged data not only unambiguously discriminated oil

from brine, a consistent and significant “attenuation” signature of evolving fluid-fluid

interface was discovered in ultrasonic data verifiable form electrical resistance

observations. Ultrasonic velocity variation and frequency dependence could be

understood employing usual anelastic/ visco-elastic models of wave propagation,

typically as Biot theory. Gassman and Biot theory applied to validate the results

within a zero frequency and megahertz range appear to offer plausible analytical

estimates of P-wave velocities. S-wave velocities are underestimated due to

ambiguities surrounding the practiced procedures and methods of quantification of

critical parameters, with no consideration to the nature of characteristic S-wave

propagation being different compared to P-wave.

ACKNOWLEDGEMENTS

Support in all respects from my supervisor Dr. Stephen D. Butt, apart from academic not to mention, has been tremendous. Without his prudent financial assistance especially, dissemination of certain critical aspects of the presented research would have been near impossible. I render academically poignant yet constructive consultations and reviews by Dr. Charles. A.

Hurich exceedingly enlightening, as they would remarkably lessen my strife in reconciling differences in mannerisms of Geophysics and Engineering. Help of Dr. Jeremy Hall at critical junctures, rather preemptive, not only made certain embroiled research issues intelligible and rational but helped stimulate thinking along fresh themes. Though rarely sought for, help of other faculty was always available whenever approached, without reservations. With all it’s stringent formal decorum all the staff at Associate Dean’s Office headed by Dr. Leonard M. Lye has been kind and candid to me, especially Moya Crocker, including assistance of Colleen Mahoney, and Nicole Parisi. Staff at Dean’sOffice, School of Graduate Studies has always been considerate in alleviating stressed situations, either circumstantial or academic, whenever requested or made aware of. Special thanks are due to all the laboratory and technical staff since their punctuality and help is vital to designing good experimental research. In this regard at Memorial University I would like to mention the names of laboratories manager Daryll Pike and staff Matthew Kurtis and Shawn Organ who always were helpful, even after hours at times. Others on technical side as Thomas Pike, Brian Pretty and David Snook helped as well. The help sought from Brian Liekens, Mark MacDonald and Blair Nickerson at Dalhousie Engineering to materialize the physical development of certain important apparatus components initially is also appreciated. Equally important to acknowledge is the role of library staff, especially Sandra Warren and Charmine Penney and colleagues, always for a timely help in seeking relevant references for helping physical realization of this document in it’s present formal form. The appropriate conclusion of the presented research substantially owes to Memorial SGS Scholarship including other generous

II

III

associated grants form NSERC, NSERC Research and Discovery, Petroleum

Research Newfoundland and Labrador, and Shclumberger, financial support since is

central to avail appropriate balance between personal and academic life. I am

fortunate to find my parents always in full confidence with me, and morally

supportive during a relatively long course of research. All my siblings have

always been morally supportive and look forward to completi

n of this research

despite certain unforeseen misfortunes.

IV

Table of Contents

ABSTRACT ... I ACKNOWLEDGEMENTS ... II LIST OF TABLES ... VII LIST OF FIGURES ... VIII LIST OF SYMBOLS ...XV

CHAPTER 1: INTRODUCTION ... 1

1.1 RESEARCH CONTEXT AND BACKGROUND ... 1

1.2 RESEARCH OBJECTIVES ... 3

1.3 THESIS ORGANIZATION ... 4

CHAPTER 2: BACKGROUND LITERATURE ...6

2.1 COMPRESSIONAL WAVES AND NEAR SURFACE ... 6

2.1.1 Historical Development of Reflection and Refraction Techniques ...6

2.2 SHEAR WAVES (S-WAVES) AND NEAR SURFACE ... 10

2.2.1 Earlier Developments and Field Trials ...10

2.2.2 S-Waves Used for Near-Surface Characterization ... 12

2.3 ELECTRICAL METHODS IN NEAR SURFACE ... 16

2.3.1 Early Developments and Practical Issues ... 16

2.3.2 Modern and Advanced Developments ... 18

CHAPTER 3: THEORY ... 22

3.1 ELASTIC WAVE PROPAGATION... 22

3.1.1 Body Wave Propagation and Definitions ... 22

3.1.2 Elastic Wave Propagation Continuum Approach ......... 23

3.1.3 Wave Velocity and Empirical Relations ... 31

3.2 WAVE PROPAGATION WITH LAYERS, FRACTURES, AND INTERFACES ... 36

3.2.1 Wave Propagation Normal and Parallel to Layers and Interfaces …………...36

3.2.2 Fracture Interface Rheology ... 41

3.3 WAVE PROPAGATION AND POROELASTICITY ... 44

3.3.1 Gassman Theory ... 44

3.3.2 Biot Theory... 48

3.3.2.1 Phenomenological Parameters ... 48

3.3.2.2 Stress Strain and Equations of Motions ...48

3.3.2.3 Dynamics and Fluid Solid Coupling Effects ...53

3.3.2.4 Application of Biot Theory ...56

3.3.3 Granular Contacts in Porous Media ...57

3.3.3.1 Principles of Contact Mechanics ...57

3.3.3.2 Implication of Contact Stiffness in Biot Theory ...59

3.4 DIRECT CURRENT RESISTIVITY METHODS ...61

3.4.1Ohmic Resistance and Resistivity ...61

3.4.2 Current Density and Potential Gradient...62

V

3.4.3 Resistivity Anisotropy... 63

3.4.4 Implications of Resistivity Anisotropy ... 64

3.4.5. DC Resistivity Conduction Method. ... 66

CHAPTER 4: EXPERIMENTAL APPARATUS AND MATERIALS ... 67

4.1 FRACTURED POROUS MEDIA CHARACTERIZATION USING S-WAVES ... 67

4.1.1 Instrumented Axial Loading Frame and Permeability Measurement ...67

4.1.2 Fractured Porous Media Specimen ...69

4.2 POROUS MEDIA FLOW AND FLUID DISPLACEMENT ...70

4.2.1 Flow Cell with Integrated Data Acquisition ...71

4.2.2 Porous Media Analogue and Immiscible Fluids ...78

4.3 DATA ANALYSIS AND INTERPRETATION ...78

4.3.1 Detectability and Repeatability ...78

4.3.2 Time Lapse Approach ...80

CHAPTER 5: FRACTURED POROUS MEDIA EVALUATION USING SWAVES……….... 82

5.1 PERMEABILITY AND WAVE PROPAGATION IN FRACTURED MEDIA ... 82

5.2 EXPERIMENTAL SPECIMEN AND TESTING PROCEDURES ...86

5.3 EXPERIMENTAL RESULTS ...90

5.3.1 S-wave Polarization and Fracture Orientation ...90

5.3.2 Fracture Spacing and Stiffness ...94

5.3.3 Permeability Anisotropy ...99

5.4 SUMMARY ...101

CHAPTER 6: IMMISCIBLE FLUIDS EXPERIMENTS - POROUS MEDIA MODEL ...103

6.1 CHARACTERIZATION OF THE DRY GRANULAR POROUS MEDIA ...103

6.1.1 Dynamic Behaviour of Intergranular Contacts and Friction ...103

6.1.2 Baseline Ultrasonic Measurements ...105

6.2 ADEQUACY AND VALIDATION OF EXPERIMENTAL MEASUREMENTS....110

6.2.1 Ultrasonic Measurements ...110

6.2.2 Electrical Resistivity Measurements ...111

6.3 ASPECTS OF THE IMMISCIBLE FLUID INTERFACE ...113

CHAPTER 7: IMMISCIBLE FLUID EXPERIMENTS - TIME DOMAIN ULTRASONIC WAVEFORM ANALYSIS ...118

7.1 EXPERIMENTS OVERVIEW ...118

7.2 RECORDED ULTRASONIC WAVEFORMS AND INTERFERENCE EFFECTS .118 7.3 VELOCITY AND AMPLITUDE ANALYSIS ...123

7.4 VP/Vs AND POISSON'S RATIO ...129

7.5 ANALYTICAL FLUID SUBSTITUTION VALIDATION ...135

7.5.1 Background ...136

7.5.2 Results and Implications ...139

VI

CHAPTER 8: IMMISCIBLE FLUID EXPERIMENTS - INTEGRATION OF

ULTRASONIC ATTENUATION AND ELECTRICAL RESISTIVITY ... 144

8.1 ULTRASONIC WAVE SPECTRAL AND ATTENUATION ANALYSIS ... 144

8.1.1 Background Theory ... 144

8.1.2 P-wave Analysis ... 152

8.1.3 S-wave Analysis ...159

8.2 ELECTRICAL RESISTANCE MEASUREMENTS ...167

8.2.1 Background Theory ...167

8.2.2 Electrical Resistance Results and Integrated Evaluation ... 170

8.3 SUMMARY... 177

CHAPTER 9: CONCLUSIONS... 178

9.1 SUMMARY ... 178

9.2 REMARKS FOR FUTURE DIRECTIONS ...181

BIBLIOGRAPHY ...182

VII

List of Tables

Table 4.1 Properties of constituents of laboratory unconsolidated core analogue .... 79

Table 5.1 An example of quantifying permeability anisotropy ...

9

Table 5.2 Various source-receiver relative acquisition azimuths w.r.t fracture plane

trajectory orientation ...

9

VIII

List of Figures

Figure 3.1. Various common wave propagation modes (a) P-wave, compression or dilatational, (b) S-wave, torsional or transverse (c) Surface , love wave (d) Surface, Rayleigh wave, modified, (Reynolds,1997). ... 23 Figure 3.2. Representation of stresses/components for unit tensor description... 25 Figure 3.3. Graphical representations of dilatation and distortion. (Tatham &

McCormack, 1991). ... 25 Figure 3.4. (a) Incident reflected and transmitted waves (rays) across a frictional interface between two semi-infinite comparable un-identical layers under well- defined compression. (b) Conceptualized alternate slip (LSL) and stick (LST) length regions due to shearing by duration of single wavelength periodicity (Miller & Tran, 1979)……….……… 37 Figure 3.5. Stress directions, orientation of coordinate directions, angels and unit vectors direction relative to the interface between un-identical elastic layers (Miller &

Tran, 1981). ... 39 Figure 3.6. Fundamental components of rock joint behavior (Bandis, 1990)... 42 Figure 3.7. (a) Nature of granular contact unstressed free and under normal force, (b) Intergranular contact under pressure (Mindlin, 1949) (c) Granular contact in presence of saturant represented with a rheological model (Biot, 1961). ... 59 Figure 3.8. (a) Direction of various contact forces acting upon a typical spherical grain (Stoll, 1989) , (b) Arrangement of spheres in a face- centered cube (Mindlin, 1949) (c) Element of volume of face-centered cubic array of spheres(d) A random set of spheres within a large number bounded by a surface under stress (Digby,

1981)……….….. 60 Figure 3.9. Graphical representations of ohm’s law, (a) resistive circuit,

(b) resistance of a material element and (c) directional/preferential resistivity

(Mooney,1980),(Reynolds, 1997). ... 62 Figure 3.10. Typical extreme cases/models of two component conductivity materials (a) rods (b) layers and (c) spheres (Grant & West, 1965). ... 64 Figure 4.1. (a) Programmable loading frame with (b) upper and lower

adjustable/tractable specimen loading platens and (c) PC-based data acquisition

module. (d) Associated DAQ card with BNC connectors(e)Ultrasonic S-wave sensor

IX

specified by letters A and B i.e.

contact face diameter and depth for any nominal size ... 68 Figure 4.2. Typical flow circuit schematic of (ASTM, D4525) air permeability

measurement method... 68 Figure 4.3. (a) Faulted reservoir rock or bedrock sandstone analogue depicting

directions of load variability (b) Thin section of quartz sandstone under cross-

polarized light (XPL) (Frempong et al., 2007)... 70 Figure 4.4. (a) Flow control system feeding the flow cell and (b) flow cell system details. ... 71 Figure 4.5. Component drawings of the porous media flow cell for (a) top and bottom end plugs, (b) leveling base, (c) copper electrical resistivity probe. (d) probe holder, (e) assembled resistivity probe, and (f) assembled flow cell. ... 73 Figure 4.6. Laboratory arrangement of immiscible fluid displacement through porous analogue in flow cell (a) Flow cell system with feeding fluids (b) Flow cell system ready for test with ultrasonic sensor position marked midspan (c) relative position of sensors

(d) relative position of components (e) retainer discs (f) plug-in end-piece and (g) base plate with adjustable damper pads. ... 74 Figure 4.7. Schematic of various components of integrated data acquisition,

(a)ultrasonic (b) electrical resistance (resistivity). ... 76 Figure 4.8. (a) Panametrics pulsar unit with the capability to transmit single or repeated pulses when internally or externally triggered (b) Four-to-two channel switch box for sequential simultaneous P- and S-wave ultrasonic acquisition

(c) Pre-amplifier (d) PCI-2 A/D card. ... 76 Figure 4.9. Components of electrical resistance DAQ. (a) Circuit schematic

(b) Digitizer (c) Programmable switch board interfaced with digitizer and I/O

(d) Electric DAQ module with power supply…... 77 Figure 4.10. (a) BioSpec soda lime (glass) spherical grains/beads compared with

(b) clean sand and (c) a laboratory soil sample. ... 79

Figure 5.1. Identification of dominant and secondary joints or cleats in a typical coal

formation (left) and the corresponding orientation of the maximum and minimum

horizontal permeabilities (right) with Khmax oriented parallel to the dominant joint

set. ... 83

Figure 5.2. Polarization/directions of S-wave in case of propagation in vertical

fractures, in (a) isometric and (b)in top view, oriented at some arbitrary direction

X

to the seismic line direction, modified, (Tatham & McCormack, 1991). ... 85 Figure 5.3. Phenomenon of splitting of an emergent S-wave in S1 and S2 components (a)S-wave splitting and polarization direction when it enters with an arbitrary

polarization in an anisotropic medium (b) S-wave splitting and its possible polarizations in response to propagation through a cracked solid with a specific

crack orientation, modified (Tatham & McCormack,1991)………... 85 Figure 5.4. (a) Cross-section of the fractured sample specimen showing sealant

application for restricting airflow. (b) Schematic of the specimen showing the location of flow ports for flow parallel and perpendicular to the fractures, and the S- wave transducers for transmitting along the specimen length. (c) Sketch showing the directions of i) parallel and perpendicular flow, ii) the transmission of

S-waves and their polarization, and iii) the normal loads applied to the fractures…. 87 Figure 5.5.(a) and (b) S-wave waveforms recorded as a function of aligned

transmitting and receiving transducers rotated with respect to the fracture plane orientation from angles of 0º through 180º with 30º increments, at Load step 1 (Hassan et al., 2013). (c) Direct stress variability upon fracture planes of fractured medium analogue with load (step 1-9), as fracture 3 is middle or half depth.

(Hassan et al., 2013). ... 91 Figure 5.6. (a) Absolute peak amplitude variation with load/stress. Top curve

for Load step 1 is obtained from ultrasonic section shown in Fig. 5.5 a , other such wave form sections corresponding to rest of the curves are not shown for same characteristic attributes (b) Velocity variation with load, VS1 corresponds to 0ᵒ and 180ᵒ amplitudes from above as more stable than VS2 corresponding to 90ᵒ amplitudes, velocity behavior facilitates anisotropy quantification (Hassan et al., 2013)... 92 Figure 5.7. Relative attenuation characteristics of transmitted S-wave displacement amplitudes with stress/load state variation when source polarization is parallel to fracture trajectory and receiver azimuthally rotated (a) receiver parallel to fracture set 0° (b) receiver at 60° (c) receiver at 90°. (Hassan et al., 2015 c) ... 93 Figure 5.8. (a) Spectral characteristics of the source pulse.(b) S-wave transmitted response spectrum of a similar intact aluminum (core). (c) Typical transmitted bandwidth when a single wave form is examined (Hassan et al., 2014 b). ... 95 Figure 5.9. S-wave spectra over the full range of fracture loads with source

polarization parallel to fracture and the receiver oriented (a) parallel to fracture set

at 0° (b) at 60° (c) at 90°(Hassan et al., 2014 b)... 97

Figure 5.10. Dilatation spectra compared for normal stress (scale) and (direction)

(Hassan et al., 2015 c)... 98

XI

Figure 5.11. (a) Permeability variation with fracture load, and (b) permeability anisotropy and S-wave velocity anisotropy correlated (Hassan et al., 2013). ... 100 Figure 6.1. (a) Measurement configuration of granular pack. (b) Velocity estimates dry and saturated granular pack (Hassan et al., 2015d). ... 106 Figure 6.2. (a), (b) and (c) P-wave ultrasonic measurements (wave forms) of dry granular sediment analogue of 25 ms duration (d) Measurement for brine saturated analogue (Hassan et al., 2015 d). ... 107 Figure 6.3. (a), (b) and (c) S-wave ultrasonic measurements (wave forms) of dry granular sediment analogue of 25 ms duration (d) Measurement for brine saturated analogue (Hassan et al., 2015 d). ... 107 Figure 6.4. Ultrasonic measurement input characteristics. (a) Input pulse (b) Input pulse spectral distribution (c) P-wave transmitted source spectrum. (d) S-wave transmitted source spectrum (e) and (f) P- and S-wave standard (Aluminum)

spectra………...108 Figure 6.5. (a) P-wave dry medium (resonance) spectrum, with power law type decay.

(b)S-wave dry medium (much pronounced resonance) spectrum, with power law type decay (Hassan et al., 2015 d). ... 109 Figure 6.6. Surface tension phenomenon depicted with possibilities of intermolecular forces and their orientation modified, (Davies & Rideal, 1963). ... 114 Figure 6.7. Depiction of interfacial monolayer formation with molecular orientation and possibilities of motion for electrochemical equilibration for two phase water based bulk immiscible systems, modified, (Davies & Rideal, 1963). ... 115 Figure 6.8. A stable and unstable immiscible bulk oil-waver interface is shown.

Eddy type features or mechanisms are shown to equilibrate energy through

momentum exchange modified (Davies & Rideal, 1963). ... 116

Figure 6.9. Contact angle (θ) as a measure wetting by spreading of a fluid on solid

surface (a) Low contact angle showing preferential wetting comparable to hydrophilic

in an aqueous case. (b) A higher contact angle showing less spreading (c) Very high

contact angle depicts fluid phobia as in hydrophobia modified (Davies & Rideal,

1963). ... 116

Figure 7.1. P-wave ultrasonograms of the immiscible displacement Test 1, with a

slow initial invading flow rate of 0.044 ml/s. Conspicuous features are marked A for

illuminated region, B for less illuminated region and C the interfacial region (Hassan

et al., 2014 a). ... 119

XII

Figure 7.2. P-wave ultrasonograms of (a) the immiscible displacement Test 3 at an intermediate initial invading flow rate of 0.11 ml/s, and (b) for Test 2 at a faster initial invading flow rate of 0.64 ml/s (Hassan et al., 2014 a). ... 120 Figure 7.3. S-wave ultrasonogram of the immiscible displacement Test 1, with a slow initial invading flow rate of 0.044 ml/s. Features of interest are similarly identified A for illuminated region, B for less illuminated region and C the interfacial region (Hassan et al., 2014 c). ... 121 Figure 7.4. S-wave ultrasonograms of (a) the immiscible displacement Test 3 at an intermediate initial invading flow rate of 0.11 ml/s, and (b) for Test 2 at a faster initial invading flow rate of 0.64 ml/s (Hassan et al., 2014 c). ... 122 Figure 7.5. P-wave (top) and S-wave (bottom) ultrasonograms of three immiscible displacement tests juxtaposed with conspicuous features of interest marked, as other features related to diffraction and interference effects also become contrastingly visible. Left Test 1 (.044 ml/s), middle Test 3 (0.11 ml/s) and right Test 0.64ml/s).

………123 Figure 7.6. P-wave velocity variation for all tests corresponding to occurrence of different possible phases (Hassan et al., 2014 a). ... 124 Figure 7.7.S-wave velocity variation for all tests corresponding to same occurrences as depicted in Figure 7.6 (Hassan et al., 2014 c). ... 124 Figure 7.8. P-wave integrated amplitudes variation for all tests corresponding to occurrence of different possible phases and identified features of interest (Hassan et al., 2014 a)... 126 Figure 7.9. S-wave integrated amplitudes variation for all tests corresponding to Same occurrences as in Figure 7.8. A higher sensitivity is indicated (Hassan et al., 2014 c)……….………….126 Figure 7.10. A conceptual linkage of Vp/Vs to subsurface description aspects (a) translated to correlation of strength properties or moduli and (b) Poisson’s ratio (Tatham, 1982). ... 127 Figure 7.11. (a)-(c) Vp-Vs cross plots of all three immiscible displacement tests for observing mutual sensitivities and anomalies. (d) Dry measurements presented with saturated measurements in similar context. ... 128 Figure 7.12. (a) Anomalies in Vp/Vs ratios variations confirm and correspond to findings of velocity and amplitude analyses, with further insight of strength aspects.

(b)A pseudo depth section presentation of the Vp/Vs ratios to exemplify a vertical

spatial evolution sense of fluids displacement (Hassan et al., 2015 b)... 130

XIII

Figure 7.13. (a) A positive Poisson’s ratio of stressed or stimulated granular material behavior when saturated, overall lateral deformation is positive. (b) Mechanical behavior under stimulation if same material was a negative Poisson’s ratio one, as overall lateral deformation tends to be negative. (c) and (d) Interparticle interfacial behavior of stressed and nonstressed positive Poisson’s ratio granular material element.(e) and (f) Interpretable interfacial behavior of a negative Poisson’s stressed ratio material, modified, (Liu, 2007)... 131 Figure 7.14. Comparable to Figure 7.12. in concept (a) anomalies in Poisson’s ratios correspond to that of Vp/Vs. (b) Pseudo depth section presentation of the Poisson’s ratios to exemplify a vertical spatial evolution sense of fluids displacement and

related strength effects (Hassan et al., 2015 b). ... 134 Figure 7.15. Experimental P-wave values of the immiscible displacement experiments compared with the analytical results generated by usual models of wave propagation based on Gassman and Biot theories. ... 137 Figure 8.1. P- and S-wave attenuation examination method illustrated, Spergen

limestone example from Toksoz et al. (1979). ... 145 Figure 8.2. Possible mechanisms causing wave attenuation (Johnston et al.,

1979)………148 Figure 8.3. (a) Time irrespective maximum energy P-wave windowed waveform.

(b) P-wave spectrum of brine saturated medium (Hassan et al., 2014 d)... 153 Figure 8.4. Standard P-wave ultrasonic spectra (a) Transmitted bandwidth (b) Dry medium response spectrum (Hassan et al., 2014 d). ... 154 Figure 8.5.Time-irrespective spectra of immiscible displacement experiments (a) Test 1 (b) Test 3 and (c) Test 2. Explained spectral curves are identified in sequence to understand aspects of attenuation, other features of interest are marked too (Hassan et al., 2014 d)... 156 Figure 8.6. (a) Time irrespective windowed S-wave waveform (b) S-wave

Spectrum for the saturated medium, spectral peculiarities w.r.t Figure 8.3 b

could be identified (Hassan et al., 2015 a)... 160 Figure 8.7. Standard S-wave ultrasonic spectra (a) Transmitted bandwidth

(b) Dry medium response spectrum (Hassan et al., 2015 a). ... 161 Figure 8.8. S-wave parametric response during immiscible displacement (a)

Initial oil spectrum. (b) Spectrum of the monolayer interfacial zone with dominant viscosity effects. (c) Spectrum of viscosity dominant evolving mixed phase front.

(d) Spectrum of instance of dominant brine flow (Hassan et al., 2015 a). ... 163

XIV

Figure 8.9. Time-irrespective spectra of immiscible displacement experiments (a)Test 1 (b) Test 3 and (c) Test 2. Explained spectral curves are identified in sequence to understand aspects of attenuation, other features of interest are

marked too (Hassan et al., 2015a). ... 164 Figure 8.10. Laser assisted sequential (a), (b) ad (c) static photographic images

depicting cross-sectional distribution of phases in a multiphase flow, illuminated oil trapped in grayer water in a matrix of quartz appearing dark (Chen & Wada, 1986)... 169 Figure 8.11. Front (left) and side (right) view of configuration and apparatus used by Lekmine et al. (2009) in their tracer solute ER profile determination in porous media flow experiment. Dimensions in centimeters are L=27.5, H=8.5, E=1... 169 Figure 8.12-(A) Previously examined ultrasonograms with features of interest clearly marked in elapsed time. Top P- and bottom S-wave ones, (L-R) Test 1, Test 3 and Test 2, respectively. (B) An example of monitoring Ohmic resistance variation against elapsed time during the immiscible displacement tests modified (Hassan et al., 2007) , (a) electrogram 1 and (b) electrogram 4. ….………….……171 Figure 8.13. (a) Electrical resistance variation with elapsed time in minutes, for

immiscible displacement Test 1 (b) Time irrespective expression of resistance

variation of same (Hassan et al., 2014 a). ... 172 Figure 8.14. (a) Electrical resistance variation with elapsed time in minutes, for immiscible displacement Test 3 (b) Time irrespective expression of resistance

variation of same (Hassan et al., 2014 a). ... 174

Figure 8.15.(a) Electrical resistance variation with elapsed time in minutes, for

immiscible displacement Test 2 (b) Time irrespective expression of resistance

variation of same (Hassan et al.,2014a). ... 176

XV

List of Symbols

a

Acceleration

A

,

Amplitudes, initial and instant

Angles of incidence and refraction

 Angular Frequency

Background or sand resistivity z

y

x , , or xyz Cartesian coordinates

Critical angle

Density

, Dissipation and dissipation scaling drag factor in Biot theory

u

or

Displacement or displacement components

, Energy (work)

or Fluid content increment in Biot theory Fluid dilation, cross coupling factor in Biot theory

Fluid dilation at constant frame volume in Biot theory

F ,

Force or force components

XVI

, Helmohlz potentials, compressional and rotational

Sx

,  ,  Horizontal fault interfacial slip and associated constants

,

,

Incident, reflected and transmitted ray

,

,

Incompressibility, dry material, saturant, and saturated etc.

Index of refraction, reflection check Index of transmission

Kronecker delta

Lame’s constant in elastic wave propagation , Lame’s constants of Biot theory Length

Mass

Knn

,

Normal and shear fault stiffness NDE Nondestructive evaluation (or NDT)



, 

Phase variables

or Porosity in Biot theory

Reflection coefficient

or

,

,

,

Rigidity dry and saturated etc.

Solid frame (U) and saturant (u) relative displacement expression

XVII

Solid-fluid coupling factor in Biot theory

Stiffness components

 , or

Stress (or shear stress) and stress components

 or

Strain and strain components Transmission coefficient

Time

, , Velocity , P-wave and S-wave ̇ , ̈ Velocity and acceleration

V

SP_,

,

,  Vertical fault interfacial slip and associated constants

Well identified structure factor in Biot theory Voltage

Void fraction

Water resistivity

Young’s modulus of elasticity

1

Chapter 1: Introduction

1.1 Research Context and Background

Near surface geophysics has increasingly advanced, and is used to provide near- surface imaging and evaluation for environmental engineering applications. In many cases, the fundamental understanding of analyses and to interpret survey results is becoming an industrial and commercial practice; and, interpretations are more qualitative than quantitative, though. Often, integrated studies utilizing two or more geophysical methods to provide independent assessment of formations are used; however, the interaction of these methods is also not fully understood or quantified. It is recognized within such community of researchers and practitioners that the fundamental research on these geophysical survey methods is needed to better implement these survey methods and to improve the quality of data and interpretations made.

This research thesis encompasses several laboratory investigations to assist with, fundamentally, designing, executing, and data evaluation of several near-surface geophysical methods that are widely used for two main environmental engineering applications. The first application is the assessment of permeability anisotropy in fractured porous media transmitting polarized S-wave, which can be applied to the optimal orienting of horizontal wells perpendicular to the direction of maximum horizontal permeability for maximization of production or injection flow rate in unconventional developments. The second application is imaging and monitoring immiscible hydrocarbon displacement in unconsolidated porous media, as for example used in remediation practices to remove hydrocarbons in groundwater by driving towards recovery wells by water flooding injection, using integrated seismic transmission and resistivity methods.

Both seismic and resistivity based methods have challenges when applied to

near-surface investigations. The research pertains to improve an understanding of

specific challenges towards better attaining of their mitigation. The conventional high

resolution seismic reflection technique, for instance, is based on inducing a seismic

disturbance at or near the ground surface and measuring the arrival times of

2

compressional or P-waves reflected from subsurface horizons. This method over the years has developed into a reliable tool for subsurface characterizations as part of geotechnical and groundwater studies. The main difficulty however is assuming resolution or control over resolution. The achievable resolution level with a high resolution reflection survey is a function of the frequency of the seismic signal.

Recovering a coherent high frequency signal can be difficult and costly. The use of shear or S-waves offers the possibility to substantially increase resolution over a conventional survey under commonly encountered subsurface conditions. In hard uniform rock, the S-wave velocity is usually about half of the P-wave velocity, but the predominant frequency is also about half, implying the S-waves may not effect resolution directly. In a heterogeneous rock or soil environment, however, S-waves can be several times slower than P-waves and have similar frequency content, implying S-waves can substantially increase resolution, towards both the subsurface structural architecture and contained saturants i.e., pore fluids. Field experiments, explained in next sections would show, the resolution obtained using S-waves was double than that obtained using P-waves, with a similar expense of resources. The results indicate that the high resolution S-wave reflection technique can be more effective in conducting subsurface investigations than using conventional technology.

Electrical resistivity relates to the bulk electrical resistance of the ground by measuring spatially varying voltages induced by the flow of electrical current between electrodes implanted at the surface. The methods are particularly sensitive to changes in the chemical content type and content concentration of pore fluids either confined or mobile, and useful for tracking such materials and contaminants, directly.

These methods can be used to find structures as faults and buried valleys.

Advancements allowing multichannel acquisition capability provides flexibility,

offering an increase in rate of DC resistivity fieldwork; as in application of electrical

resistivity tomography for investigation of morphologically complex subsurface

environments. The resistivity method is the underlying concept of the induced

polarization (IP) method in data collection, but the IP method involves analysis of the

length of time the earth remains disturbed electrically after the disturbing function has

been removed. In an electronic sense, the earth’s discharge rate is similar to that of a

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capacitor. The rate of decay of the induced voltage is dependent on ion mobility in the charged volume. The ions in clays, for example, are highly mobile. Measurements can be made either in the time domain, with voltage as a function of time, or in the frequency domain, where the phase delays of various frequencies are measured. This method was initially developed for sulphide mineral exploration and has been used in for groundwater exploration. The study thus demonstratively signifies the attainment of resolution for unambiguous description of near subsurface anomalies, especially fractures and immiscible fronts, by integration of data, in acquisition and inferring, not well addressed for it’s time.

1.2 Research Objectives

As described above, this research is focused on fundamental laboratory investigations of i) evaluating relative horizontal permeability in fractured porous media for the optimal orientation of horizontal well bores, and ii) monitoring immiscible hydrocarbon fluid displacement in unconsolidated sediments using integrated seismic and resistivity methods. In this context, the specific objectives of this research include:



Development and validation of laboratory facilities and data analysis methodologies to conduct the investigation.



Evaluation of S-wave polarization as a means to evaluate permeability anisotropy, in particular, the ability to resolve the ratio of maximum to minimum permeability and their directions

fracture stress variability.



Evaluation of transmitted P- and S-waves to differentiate between hydrocarbon and brine saturation, including mixed saturation in the immiscible contact zone.



Evaluation of DC electrical resistivity to differentiate between hydrocarbon saturation, brine saturation, and relative mixed saturation.



Evaluation by Gassman and Biot theory to determine fluid properties for

hydrocarbons and brine.

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1.3 Thesis Organization

This thesis is organized in 9 chapters, making conclusions the 9th. Chapter 1 introduces the research context and motivation for the research, including a brief summary of some of the challenges and limitations with current practices and data analysis methodologies.

Chapter 2 forms the background literature review, and consists of three distinct parts. Each part provides to understand the application of a specific geophysical method to engineering and environmental problems, focused on near- surface applications. Part 1 focuses on P-wave applications, Part 2 on S-wave application, and Part 3 on Electrical methods.

Chapter 3 includes theory pertinent to both implement experiment design and data analysis and interpretation. It includes fundamental definitions of body waves i.e., P- and S-waves, and the theory of elastic wave propagation through isotropic elastic media including wave based and ray based principles. This is extended to anisotropic media, where specific case of elastic wave propagation in layered fractured anisotropic media is examined, and then to poroelasticity for wave propagation through saturated porous media including scattering phenomenon, particle contact mechanics and theories, and frequency restrictions. Finally, fundamentals of DC resistivity methods are detailed.

Chapter 4 describes the laboratory infrastructure and facilities developed to conduct the research, including the various types of material analogues used to simulate subsurface conditions and formations. Experimental measurement accuracy and repeatability are also discussed.

Chapters 5 through 8 comprise the systematic presentation of experimental results and interpretations from the laboratory investigations. Chapter 5 focuses on the fractured porous media investigation

through transmitted S-waves, while Chapters 6 and 7 are focused of the immiscible displacement study imaged with integrated data.

Chapter 6 also highlights the adequacy and validity of relevant experimental

procedures and consequent acquired data where the quantitative validation of the

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velocity (for ultrasonic P- and S-wave) results are provided in chapter 7. Only P-wave results of the immiscible displacement experiments are deemed appropriate or sufficient for discussion. All the results in these three chapters consist of components of specific relevance to application of geophysics for engineering and environmental problems.

Chapter 8 assumes a further validity of the results in conceptual relevance to the Gassman and Biot theories with some sources of ambiguities and uncertainties.

Conclusions are summarized in Chapter 9.

A detailed list of symbols is provided in the start of the thesis with sufficient

explanation thereof to adhere as much as possible with the conventional symbols of

discussed concepts and theory, however where ever required adequate explanation in

the list and in the text should alleviate any misrepresentation or confusion.

Chapter 2: Background Literature

2.1 Compressional Waves and Near Surface

2.1.1 Historical Development of Reflection and Refraction Techniques

Early or earliest seismological methods were focused on the analysis of waves generated by naturally occurring earthquakes and related seismic events. Mallet (1848, 1851) was a pioneer in using an artificial energy in the form of black powder and a bowl of mercury as a detector. He could record only very low velocities due to sensitivity issues, however. Abbot (1878) later measured P-wave velocities using essentially the same type of detectors and a stronger explosive source. Milne and Gray (1885) were the first

use a seismic receiver spread employing two inline seismographs and explosive and falling weight type energy sources. Otto Hecker (1900) used nine mechanical horizontal seismographs in line to record both P- and the S-waves. Further defining the subsurface conditions by making use of the seismographs was put forward by Milne in 1898 (Shaw et al., 1931). Ludger Mintrop in Germany in 1914 devised a seismograph which could record explosion created seismic waves making seismic exploration a feasible proposition.

L.P. Garret in 1905 suggested the use of seismic refraction to find salt domes however at that time suitable instruments had not yet been developed (De Goyler, 1935). Later during the period of 1920 and 1921, Mintrop did some refraction work running lines across known features and discovered the Meisendorf dome, a step forward in establishing the foundations of the technique. Much of this early work was applied to the detection of deeper targets for mineral and oil and gas exploration - additional historical development is outlined in Sherrif and Geldart (1982).

Shallow targets are where the use of the seismic methods for engineering studies may be done such as groundwater exploration, waste site evaluations, industrial mineral and metals exploration and exploration for certain energy materials. Evison (1952) working in New Zealand was first to point out that the region of the earth that had been the least successfully explored by means of elastic energy was the first few

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hundred feet below the surface. He observed that the standard seismic techniques developed in the course of the search for oil at greater depths were relatively ineffectual at shallow levels, and concluded that the low resolving power of the techniques of the day were due to the type and the nature of the method

the energy source used for the most part. For shallow surveying, an energy source capable of generating an impulse of desired frequency and duration was really required. He outlined and identified mainly the problems and frustrations of working in the shallow environment, but still contributed towards ensuring success in future works. The research by Pakiser at al. (1954) at the U. S. Geological Survey pioneered the method using a specially constructed portable shallow reflection seismograph. On the basis of several experiments performed in different areas with significant rigor, the usefulness of high frequencies or high-frequency seismic reflection systems was very clearly demonstrated for mapping both

shallow horizons (Pakiser & Warrick, 1956). They further expected that the method should help solve a variety of ground water problems involving depth to bedrock, shallow structure (artesian water), and, under certain circumstances, aquifers within unconsolidated materials may be imaged with the help of the shallow reflection seismograph. Similar uses were suggested in engineering geology, for example, finding bedrock depths in permafrost areas, where the high near surface permafrost velocity disqualifies the seismic refraction method, and in addition was suggested finding shallow structures for the natural gas storage near large cities. Mooney (1980) provides sound fundamental knowledge of the application of the shallow seismic methods, including analysis and interpretation for identifying two layered structures, three layered structures and non-horizontal multiple layer structures.

2.1.2 Near-Surface Methods

Modern use of shallow seismic reflection methods began with Schepers(1975) who produced some excellent shallow P-wave reflection results. His area of interest and definition for near surface investigations appear to be few meters to a maximum

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of 100 m of the earth’s crust. His investigations were aimed at solving engineering problems, special focus being upon civil engineering. He had divided the measurements into two categories i.e.

for investigation of subsurface cavities and, for hydrological and rock mechanics studies. He experimented with various kinds of sources to determine and examine the frequency band produced. The sources used were different types of hammer or thumper and an electric sparker buried in a water filled bore hole. The acoustic signals generated were received at a distance of 2 m from the source. At that distance of 2 m the recorded frequency band ranged from 50 Hz to 800 Hz width. The dominant frequency recorded was 150 Hz and the duration of the pulses recorded were all about 15 ms. Technically speaking their finding manifested that the two way travel time for the seismic wave within the near surface shall be smaller than the duration of the energy pulse itself for most surveys. An example of survey geometry was illustratively offered where the reflector of interest is assumed to be at 50 m depth and the geophone spread is 25 m. It is suggested that all the reflected energy either P- or S-waves should be recorded within the 50 to 100 ms time window depending upon the velocity distribution.

Hunter et al. (1982) enunciate about the impact of the introduction of multichannel digital engineering seismograph system upon the scope of applications of seismic methods to near surface geologic problems. They attribute it to the advent of much economical computing power, in addition. Special emphasis is drawn upon the instance of use of such system as described by (Hunter, 1980) and (Hunter et al., 1980) in allowing simple applications of reflection seismic technique, be used in a variety of applications where high resolution subsurface structural information is required. They discuss the application of the “optimum window method” in an initial field testing. The essence of the “optimum window” technique is in ensuring a correct selection of source-geophone geometry to observe a shallow reflector using an array of twelve geophones. Special significance lies in application and testing of the technique at several field sites in Canada with differing geologic settings. The tests showed the optimum window technique to be extremely effective in areas of thick unconsolidated overburden with the target as the bedrock surface. Hunter et al. (1984)

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in their shallow seismic reflection mapping of the overburden-bedrock interface studies draw due emphasis upon the issue of detectability, suitability of conditions and its impact on attainable resolution. They delineated the unconsolidated overburden bedrock interface and the heterogeneities within the overburden to a depth of 20 m.

Refraction seismics developments for near surface investigations identify generalized reciprocal method (GRM) both a primary and significant contribution (Palmer, 1981). The important aspect is introduction of concept of the refractor velocity analysis function, the generalized time-depth, the optimum XY value and the average velocity. GRM is the generalized technique for processing and interpreting in-line seismic refraction data consisting of forward and reverse travel times. This method is also used for surveying undulating targets. The inline seismic data is used and consists of only forward and reverse travel times. For refractor velocity analysis and time depth calculations, the travel times used are recorded at two geophone positions which are separated by a variable distance or spacing XY. Upward traveling segments of the rays received to each geophone emerge from near the same point on the refractor for an optimum XY spacing, which offers two advantages, making velocity analysis simple and the time-depth showing the most detail. Useful when the overlying strata have velocity gradients, since the depth conversion factor is relatively insensitive to dip angles up to about 20 degrees.

Lankston (1990) provides a classical example of the use of refraction method for shallow or near-surface target investigations. He concludes that in the specific case of a waste site evaluation and in ground water exploration, i.e.

cases in which the appropriate study requires high vertical and lateral resolution of near surface targets, the refraction seismic method is one of the most powerful geophysical tools provided appropriate procedure is adopted during field operations. It is argued that refraction seismic method could bear much useful information provided the resolution required is well afforded and appreciated by the interpreter, which at times reflection seismics cannot provide in the near surface, economically and efficiently in comparison. Consequently, the respective method must be selected when it is the

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obvious choice. General criteria for selecting the refraction method and GRM data processing are often met in groundwater exploration and waste site evaluations which are a) the target is shallow b) lateral velocity and dip changes on the target are expected or are themselves the anomaly of interest, and c) there are only a few targets are of interest.

2.2 Shear Waves (S-Waves) and Near Surface

Not much research work has been reported specifically where S-waves have been rigorously studied in laboratory for near-surface investigations, in fact, much of the S-wave investigations work reported have been in oil and gas exploration serving specific interests only. This section details literature focused on S-waves and their propagation that are most relevant for the thesis research, including characterization of anisotropy, improved illumination with lateral and vertical resolution, SH and SV polarizations, sensitivity to velocity gradients, attenuation, dependence of resolution upon source type, and appropriate geophone spacing and array geometry.

2.2.1

The earliest but rather indirect and speculative observation of the S-waves or transverse waves is reported in Horton (1943). A P-wave well velocity measurement survey was done by Shell Oil incorporated in Vernon Parish, Louisiana. Two shots were taken out side of the well at 960 and 1046 feet distances while the seismometer or the geophone was placed inside the well. The examination of the seismograms was reported also to show other well defined events than the primary events. These secondary events were assumed as transverse waves. There potential was quickly realized technically, in enabling to determine the properties of some zones of interest consisting of clays and sands. It was also established that these waves could not be generated either at or close to the shot points, so it was quite evident that they would have emerged or reflected from the interfaces as some kind of converted mode. Horton (1943) was not convinced however if there was ample evidence available

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whether the secondary arrivals treated in the analysis were transverse waves originating from reflection of compressional waves at the base of the weathered layer or they were transverse waves originating at the shot point. The computations based on the premise that they were the former, resulted in obtaining plausible values for the elastic constants and for the variation of these quantities with depth. Indirectly thus the potential of multi-mode acquisition and the potential of transverse waves in probing the near-surface weathering above bedrock was identified but not very well appreciated, perhaps.

Ricker and Lynn (1950) conducted a multi-component acquisition survey where an interpretation revealed the identification of the mode converted S-wave’s potential as a tool for improving resolution and delineating subsurface structure. In studying the arrival times a prominent and well isolated disturbance was observed.

The arrival time of events occurred in the comparatively quiet region with little noise between the arrival of the primary disturbance i.e.

dilatational or P-waves, and the arrival of the ground roll, thus the disturbance was well isolated. Two types of geophones were used or installed, vertical-component and horizontal- component. Such disturbances as strong signals were received by horizontal-component geophones which were installed with their direction of response along the line from the shot point, but were scarcely at all by vertical- component geophones. It was suspected as being a disturbance which had started transmitted as a wave of dilatation or P-wave, but had been detected as a shear wave. The evidence considered was the horizontal in-line motion of the received vibration and the time of arrival of the disturbance.

Since these early studies, research for improving understanding of shear waves had been almost always on two lines, firstly from the point of view of designing an adequate source to generate the shear wave energy of desired characteristics economical and consistent with the nature of problem at hand i.e.

band width and geologic issues, and secondly appropriate acquisition of the energy/signal transmitted from the subsurface to map the subsurface where most experiments if not all have been practically exploring or probing the near surface. One of the important

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contribution on similar lines is a series of studies described in Cherry and Waters (1968) and Erickson et al. (1968) on the development of a shear wave vibrator and recording system suitably designed specifically to be used for well planned experimental surveys. The system designed would enable to generate P and SH modes with reasonable control over the selected band width.

It is quite interesting to note that not only that the peculiar characteristic behavior of the S-wave propagation was identified but from the perspective of a broader research the use of elastic shear waves in the earth to supplement the information already being gained from compressional waves reflection and refraction methods was considered attractive too. The facts led to believe that it would add new knowledge about the sedimentary section/s by recording shear wave with “suitable travel paths”. Cherry and Waters (1968) were somewhat following up on the works of Jolly (1956), however they adopted continuous signal technique in their acquisition.

In simple terms however their presentation and/or interpretation of the times series or data was based on cross-correlogram development and examination consistent with the same scheme explained by (Crawford et al., 1960).

Another important feature in the research appeared to be identifying and signifying the need for an appropriate terminology for defining the transverse or S- waves. Cherry and Waters (1968) tend to define it as a disturbance which shall move through an infinite medium that the displacement of the point is parallel to the wave front in distinction to the P-wave where the displacement to the point is perpendicular to the wave front. They confirmed P-wave velocity is always higher than the S-wave velocity and both velocities are controlled by different kind of elastic moduli of the medium. It appears that in their attempted field work they would consider only the SH mode as a legitimate S-wave.

2.2.2 S-Waves Used for Near-Surface Characterization

Relatively recently, in developmental context, attention is drawn more

towards importance of correct and improved interpretation of S-waves compared to

highlighting phenomenological aspects of wave generation and propagation ( Dohr

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& Janle, 1980). Dohr & Janle (1980) define the Vp/Vs as velocity ratio and regard it as an extremely important parameter not fairly used for the time. Velocity ratio’s sensitivity is said to be ascribed to its being proxy descriptor for the elastic constants.

The importance of Vp/Vs is further highlighted referring to some experimental results, done mostly for less than 1 km depths, where it differs from the value for a typical poisson’s body value of √3. The difference from the typical value of √3 in subtleties provided details about the degree of consolidation, porosity and type and/or degree of saturation variation. Further attention was drawn toward where it could be useful in identifying/picking weathering zones too, unlike the sediments for which its value is around 2, as for weathering zones it should be a high value. One important inference drawn from observations/record examinations was that in the case of sediments, S-wave absorption is greater than that of the P-waves and on the analogous grounds it could be deduced that the absorption of SS waves would be greater than the PS waves since for the latter the significant part of travel path is for P-wave mode. A higher sensitivity of the pure SS waves to the velocity gradient than that of PS and P waves was also observed. This manifest was explained on the basis of the path distance travelled by SS, PS and P waves, for SS path would be longer than the P-wave which would be least sensitive and on the other hand PS wave being less sensitive to velocity gradients than the S-waves.

Helbig and Mesdag (1982) provide a detailed account on various

developmental aspects of S-wave surveys, and highlighted that S-waves are very

useful in imparting desired resolution in aspects of manner the S-waves tend to

deform the medium which it propagates through. Stumpel et al. (1984) examined the

usefulness of shear waves in determining sediment parameters of such materials as

peat. The good penetration of shear waves in partially saturated or gas-containing

sandy material was reported as useful for assessing such materials compared with

compressional mode suffering high absorption. Either for a natural resource or from

construction perspective subsurface evaluation on the basis of differences between P-

wave and S-wave absorption is identified i.e., l/Qp and l/Qs. It is also identified that

Qp/Qs versus (Vp/Vs)

may be used for a thorough description of lithological and

hydrological properties of sediments. In concluding the usefulness of the shear wave surveys Stumpel et al. (1984) summarize various near surface related problems or situations where reliable solutions could be sought using S-waves. These include lithological boundaries inside an aquifer, clear delineation of the base of an aquifer, location of cracks and voids and fault zones, with assessment of their shear moduli.

The significance of conducting combined P-wave and S-wave surveys for the near surface had prior been highlighted too, by (Imai et al., 197

). Imai et al. (197

) suggested that such practice should provide useful data enabling appropriate understanding of site condition in terms of the dynamic parameters or characteristics used in various engineering studies. Imai et al. (1977) report on a rigorous and thorough survey done almost throughout the Japan where the technique was a sort of modified refraction method especially suited to soil characterization. In employing the technique especially designed geophone would be attached/clamped inside the bore hole ranging from one to two meter in depth, and a weight drop, hammer or a small shot-fire type source could be used suggesting a direct arrival type of acquisition performed. Around 242 of such experiments were done. The final data determined as P-wave and S-wave velocity values for the sites was correlated to various mechanical properties of the soil where a reasonable agreement was reported.

However the most reliable empirical relation sought was between the N-value of the standard penetration test or the unconfined compressive strength and the shear wave velocity values.

Prior to the combined S-wave and P-wave tests performed in the Japan as explained above, testing of such methodologies while proceeding with US Geological Survey and the UK National Coal Board cooperative research initiative is also reported in (Hasbrouck & Padget, 1982). The principal method to be tested for investigations was “Shear Wave Seismics”. The goal appeared to be devising a seismic method for the exploration of coal in the area, at least initially. In order to find the useful methodology for prospecting firstly, they had to test their equipment for adequate performance and sensitivity, a shear wave search survey, so, for a shallow structure was done. In this tuning up procedure sensitivity of the S-wave

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signals to the change in the polarity of source and/or the geophone alignment/geometry was examined. A comparative analysis of the reflected P-wave and S-wave velocity was also performed. Identification of SH refracted and reflected arrivals were made. Issue of attaining reasonable temporal resolution was also addressed by examining S-waves for identifying fine enough discontinuities. The interpretation overall turned out a success for the clay bed surveyed as flat laying beds within and immediately below the clay could be identified, along with the discontinuous horizons further below.

The second shear wave survey was for locating a coal subcrop, with refraction method. Some prior knowledge of the geological and stratigraphic detail of the area helped, especially when the depth of the overburden was available, design an adequate geophone array. The time difference between the refractions through the sandstone above and below the subcrop could be used to determine the depth of the subcrop. The coal bed/seam on the roof and foot side would be bounded by the sandstone layers. The layers or stratum of the sandstone below the seam would terminate and would not extend to the end of the subcorp, thus allowing such an anomalous situation where boundaries i.e., boundary limit effect, could be inferred delineated after adequate data acquisition planning. The third study in this testing and evaluation project was doing a survey for an old abandoned mine. The discussions of results are deducible to confirm that, even for a speculative interpretation, velocity and amplitude change in the signals was reported as interacting with structural anomalies. Frequency response to cavity was termed as “resonating” character of the trace. And the issues of resolution were also found where the transmission of SH was described as not being able to “pass” i.e., see, a feature at times.

In summary, the usage of S-wave surveys for near-surface characterization had developed and been applied to several special situations requiring significant resolution. These include lithology delineation, resolving acoustic transparency or

“blindness”, delineation of cracks and fissures, faults, fault orientations, preferential

alignments of geological units/members, barriers, and acoustic contrasts.

2.3 Electrical Methods in Near Surface

Electrical methods normally investigate to depths limited by the geometry of the electrical probe array and are therefore well suited for near-surface investigations (Auken et al., 2006). They have been applied to near-surface geological mapping, waste-site characterization, plume delineation, hydrogeological mapping for the spatial extent of aquifers and their degree of vulnerability, exploration or unconsolidated materials such as gravel, sand, limestone, and clay, geotechnical investigations of building and road construction sites, and location and identification of subsurface utilities, unexploded ordnance, and many others. This section will give a brief summary of their application for near-surface characterization

2.3.1 Early Developments and Practical Issues

Rust (1938) provided developmental details of the early history of electrical methods for prospecting and exploration. In 1830, the first person ever to identify subsurface, or more precisely, exploitable electrical properties for exploration purposes in ore deposits was R.W. Fox. Fox proposed that “electric currents and potentials were associated with certain ore deposits in Cornwall”, where he meant occurrence of natural electric potentials. Early electrical probes were highly sensitive to near-field effects, and Ambronn in 1913 reported a two- electrode method in an application of well logging where the drilling fluid in the well served as an unaltering medium in the near vicinity of the electrodes, with one electrode fixed and the other being the tool itself. A modification of the two electrode method was introduced by Schlumberger in 1912 with the development of equipotential line maps and later improved by others (Lundberg & Nathorst, 1919) and patented (Nichols & Williston, 1932). The general idea was that the displacement from the predetermined regular equipotential “characteristic lines”

for a constant/in-varying earth would determine the deviated but constant horizons of equivalent potential lines depicting subsurface anomalies, for offering improved interpretation.

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